Conventionally, MW sensors are known (e.g., Japanese Laid-Open Patent Publication No. Hei 7-37176 (1995)) as one form of a crime prevention device in which microwaves are transmitted toward a protected area and, when a human body is present in the protected area, the human body (intruder) is detected by receiving the reflected waves (microwaves modulated due to the Doppler effect) from that human body.
Moreover, as one type of MW sensor, sensors are known in which a distance to an object is measured by using microwaves with two different frequencies. This type of sensor is set up so that microwaves with two different frequencies are transmitted toward a protected area, and the phase difference is detected between two IF signals based on the reflected waves of each of these. This phase difference correlates to the distance to a target (an object to be detected such as a human body). In other words, it is possible to measure the distance to the target by obtaining this phase difference. The following is a description concerning an operation for detecting the phase difference in IF signals with this type of sensor.
When the IF signals, which are based on the reflected waves of microwaves with two different frequencies, are sine waves IFout1 and IFout2 (with a phase difference corresponding to the distance to the target) as shown in FIG. 4(a), the rectangular waves A and B formed from these IF signals are as shown in FIG. 4(b). It is then possible to measure the distance to the target by detecting the phase difference (the phase difference Δt at the rising edge portion of the rectangular waves in FIG. 4(b)) between the rectangular waves A and B.
However, this type of sensor has the following issues due to the fact that the distance to the target is obtained by using only the phase difference of two IF signals. FIG. 5 shows the relationship between the absolute value of the phase difference (hereafter, simply called “phase difference”) of the two IF signals of this type of sensor and the distance to the target. As shown in FIG. 5, the line indicating the relationship between the phase difference and the distance to the target is folded over at the point of a 180-degree phase difference. For this reason, two points are obtained as the distance to the target that is obtained by the detected IF signal phase difference when obtaining the distance to the target by using only the phase difference of two IF signals. In FIG. 5 for example, when the phase difference of the two IF signals is 60 degrees, distances of 5 m and 25 m are obtained as the distance to the target. That is, there is no unique relationship between the phase difference of the IF signals and the distance to the target that is obtained by using that difference, and therefore, it is impossible to accurately obtain the distance to the target by using only the IF signal phase difference. Obtaining two distances with a single phase difference in this way is generally referred to as “fold-over error.”
In the case of applying this type of sensor as a crime prevention sensor, for example, when the area that is intended to be protected is of a range of 10 m, that is, when there is no need to detect for human bodies present in positions farther than 10 m away, the phase difference of the two IF signals is detected as 60 degrees in the event of a human body being present in a position 25 m away from the sensor due to the existence of the “fold-over error.” In this case, there is the possibility that the sensor discriminates the distance to the target to be 5 m, and issues a false alarm in which an object detection signal is generated (alarmed).